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On my '09 Aveo I have permanently blocked the lower grille with no adverse effect. For the upper grille I made a removable grille block that I use only in winter. I also have permanently installed a under belly pan. When I bought the car as new there was nothing at all there - everything in front and underneath was one big, wide open area: no aerodynamic benefit, no heat retention.
Remember, when you are stuck in traffic your car isn't moving, but it IS producing heat. Heat rises, and something needs to be left open for adequate ventilation in warm weather, so it's better to keep open the upper grille. If everything is blocked, you will tend to run too hot in summer. If the frontal area is just one big, wide open space you will wait a long time in winter for the car to reach operating temperature and your FE will suffer. It's a balance, or a trade off.
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